Category

Route Distinguisher We know VRFs allow IP address space to be reused among isolated routing domains. For example, assume you have to connect to three customer sites, all of which are using 192.168.10.0/24 as their local network. We can assign each customer its own VRF so that the overlapping networks are kept isolated from one [...]

When BGP is not behaving correctly, a “trick” to temporarily stop peering with a neighbor is to use the following command: router bgp 194 neighbor<ipaddress> password xxx Since the other router doesn’t have the same password, the two routers will stop talking to one another, without you having to do anything else. Later, when the [...]

A transit AS is an AS that routes traffic from one external AS to another external AS The “show ip bgp” command is used to display entries in the BGP routing table. The AS-PATH attribute is used to prevent BGP routing loops. When receiving an BGP advertisement, the router checks the AS-PATH attribute, [...]

Routing policies for a peer include all the configurations such as route-map, distribute-list, prefix-list, & filter-list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be cleared, or reset, for the new policy to take effect. There are two types of reset, [...]

In BGP it is a component that is used to modify the admin distance of eBGP in order for an interior portal routing protocol (IGP) to take precedence over an eBGP route. By default,External BGP (eBGP) has an admin distance value of 20. Administrative distance is the first criterion that a router uses to determine [...]

BGP (Border Gateway Protocol ) supports authentication using the md5 (message digest 5) algorithm. For authentication to be successful md5 authentication should be configured on both sides. When authentication is enabled when any TCP segment belonging to BGP exchanged between peers is accepted only if the authentication is successful. router bgp 100 no synchronization bgp [...]

If Border Gateway Protocol (BGP) is not propagating any internal BGP (iBGP) learned routes to another iBGP peer because it requires a full iBGP mesh within the Autonomous System (AS). To overcome this issue you can configure BGP router as a route reflector By configuring a BGP router to be a route reflector, a full [...]

By default, fast external failover is enabled for eBGP neighbor. So when the egress interface fails, it immediately bring down the BGP session and next best path will be selected. if you have it disabled, it will rely on the BGP holdtimer.

In the multihomed environment load balancing is not an option because BGP select only one best path to destination when the BGP routes are learned from different AS’s.To achieve this we need to setup a better metric for the routers in the range 0.0.0.0 to 128.0.0.0 that are learned from ISP A and better metric [...]

There are three types of BGP Routing tables: a) Adj-RIB-in b) Adj-RIB-out c) Loc-RIB Adj-RIB-in stores the unprocessed information received from its peers. Here the best path selection occurs as per BGP attributes and after conformation path is entered into the local bgp table i.e Loc-RIB. From the local RIB table it conform the next-hop [...]

SoO for BGP is “linked” to CE-neighbor. So, when a prefix needs to be advertised to a CE neighbor, we check the SoO of the prefix with the SoO of the BGP neighbor. For anything else, it is linked to interface. The configuration can be done in four ways –“route-map in” on CE BGP neighbor [...]

The bgp cluster-id command is used to assign a cluster ID to a route reflector when the cluster has one or more route reflectors. Multiple route reflectors are deployed in a cluster to increase redundancy and avoid a single point of failure. When multiple route reflectors are configured in a cluster, the same cluster ID [...]

AS-PATH prepending is an important tool in BGP toolbox to influence the return traffic back to your networks.AS-PATH prepending changes the way that your advertised prefixes appear to the rest of the Internet. The as-path length is evaluated early on in the path select process so this is a powerful tool to use with BGP. [...]

EBGP next hop is not changed if the BGP next hop in the BGP table belongs to the same IP subnet as the EBGP neighbor to which the update is sent. This rule ensures optimum packet forwarding in partially-meshed EBGP deployments (example: internet deployment). You can disable the EBGP next hop optimization with neighbor next-hop-self [...]

update-source is typically used on iBGP sessions, that are generally multihop and between router’s loopback addresses. I often see people configuring update-source on directly connected eBGP session but this is by no mean needed. For eBGP multihop session, it might be needed if you have multiple paths to destination.

BGP attempts to obtain a router ID in the following ways (in order of preference): By means of the address configured using the bgp router-id command in router configuration mode. By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved loopback address configuration. By using [...]

In BGP there is rule called Split Horizon Rule as per the rule that any route received from an iBGP neighbor must not be advertised to any other iBGP neighbor. This loop prevention mechanism induces a requirement that all iBGP routers in the network should be connected in logical full mesh topology to allow for [...]

A community is a group of prefixes that share some common property and can be configured with the BGP community attribute. The BGP Community attribute is an optional transitive attribute of variable length. The attribute consists of a set of four octet values that specify a community. The community attribute values are encoded with an [...]

BGP synchronization was an old routing loop prevention mechanism that worked in the following way: A BGP border router would not propagate a BGP advertisement to an eBGP peer until the router does see the same IP prefix learned in IGP ( OSPF, ISIS or other internal protocol). This would ensure that there are no [...]

A route reflector is BGP router that is allowed to break the iBGP loop avoidance rule. Route reflectors can advertise updates received from an iBGP peer to another iBGP peer under specific conditions. By breaking the rules, route reflectors are used to eliminate the full mesh requirement and allow for building iBGP networks that scale [...]

A BGP-speaking router does not discover another BGP-speaking device automatically. A network administrator usually manually configures the relationships between BGP-speaking routers. A BGP neighbor device is a BGP-speaking router that has an active TCP connection to another BGP-speaking device. This relationship between BGP devices is often referred to as a peer instead of neighbor because [...]

BGP was designed to carry only unicast IPv4 routing information. BGP configuration used the Network Layer Reachability Information (NLRI) format CLI in Cisco IOS software. The NLRI format offers only limited support for multicast routing information and does not support multiple network layer protocols.

-Prefer the path with the highest WEIGHT. Note: WEIGHT is a Cisco-specific parameter. It is local to the router on which it is configured. -Prefer the path with the highest LOCAL_PREF. Note: A path without LOCAL_PREF is considered to have had the value set with the bgp default local-preferencecommand, or to have a value of [...]

A fully meshed iBGP network becomes complex as the number of iBGP peers grows. You can reduce the iBGP mesh by dividing the autonomous system into multiple subautonomous systems and grouping them into a single confederation. A confederation is a group of iBGP peers that use the same autonomous system number to communicate to external [...]

There are two main types of AS: Stub AS: This is an AS that is connected to only one other AS. It is comparable to a cul-de-sac (dead-end street) in our road analogy; usually, only vehicles coming from or going to houses on the street will be found on that street. Multihomed AS: This is [...]